The invention relates to laser beam attenuators and, more particularly, laser beam attenuators using transmissive or reflective interference filters such as multilayer dielectric filters. This invention is the result of a contract with the Department of Energy (Contract No. W-7405-ENG-36).
Intensity control is a problem facing many laser users. For example, it can be important to determine and know optical damage thresholds in optical systems and to maintain beam strength below such thresholds. This is typically accomplished with output beam attenuation devices.
Two commonly used devices for attenuating laser beams are polarizers and neutral density filters. A set of wedged plates can be used to generate Fresnel reflection losses, too. None of these techniques are well suited for use with unpolarized, relatively high power lasers such as excimer lasers.
Polarizers are available in a wide assortment of structures and provide, by rotation with respect to linear input polarization, simple and efficient beam attenuation for polarized and collimated input beams. Problems arise, however, if polarizers are used with excimer lasers which produce unpolarized and highly divergent output beams. Collimating such beams adequately for small polarizer acceptance angles is difficult or impossible. Too, polarizers reduce the energy content of unpolarized laser beams on the order of one half. Usually, such large amounts of attenuation are not desired.
Neutral density filters are generally inconel-coated glass or fused silica and are not limited to use with polarized or collimated beams. However, because they operate by absorbing part of the incident radiation from the beam, such filters have very low ultraviolet damage resistance. Damage thresholds of less than 0.05 Joules per square centimeter (J/cm.sup.2) at 248 and 351 nanometer (nm) wavelengths have been measured on this type of filter. Furthermore, even at low fluence levels, metallic neutral density filters are subject to changing optical density values during long periods of use.
Wedge plate attenuators may comprise, for example, two pairs of counter rotating quartz wedges. Such attentuators produce excessive losses when used with unpolarized light and provide limited apertures. Also, the wedges cause beam expansion in the plane of incidence that may be unacceptable in some applications.
These and other problems of the prior art are addressed by the present invention, wherein a laser attenuator is provided for unpolarized laser beams which have low energy absorption during beam attenuation.
One object of the invention is to provide variable and selective attenuation for laser beams.
Another object of the present invention is to selectively attenuate unpolarized laser beams.
Still another object of the invention is to provide selective attenuation for polarized laser beams.
Yet another object of the invention is to provide continuously variable laser beam attenuation.